1. Multiplexing
• The combining of two or more information
channels onto a common transmission
medium.
• Basic forms of multiplexing:
– Frequency-division multiplexing (FDM).
– Time-division multiplexing (TDM)
– Code-division multiplexing (CDM)
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2. FDM
• Frequency Division Multiplexing
– The deriving of two or more simultaneous,
continuous channels from a transmission medium
by assigning a separate portion of the available
frequency spectrum to each of the individual
channels.
• FDMA (frequency-division multiple access):
The use of frequency division to provide
multiple and simultaneous transmissions.
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3. • Transmission is organized in frequency
channels, assigned for an exclusive use by a
single user at a time
• If the channel is not in use, it remains idle and
cannot be used by others
• There are channeling frequency plans
elaborated to avoid mutual co-channel and
adjacent-channel interference among
neighboring stations
• The use of a radio channel or a group of radio
channels requires authorization (license)
– for each individual station or for group of stations
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4. FDM (Frequency Division Multiplexing)
Power
Frequency
FDMA
Bc
Bm
Time
Frequency channel
Example: Telephony Bm = 3-9 kHz
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5. FDD
• Frequency division duplexing
– 2 radio frequency channels for each duplex
link (1 up-link & 1 down-link or 1 forward link
and 1 reverse link)
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7. Receiver response
• Fundamental channel
• Spurious channels
– Intermediate
frequency
Ideal – Image frequency
1.2 – Channels received via
1
LO harmonics
Response
0.8
0.6 – Intermodulation
0.4
channels
0.2
0
1 2 3 4 5 6 7 Frequency
8 9 10
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8. Intermodulation
• 2 or more signals, nonlinear circuit
• Intermodulation products: Fi = CkFk
• {Ck} positive/negative integers or zero
• {Fk} frequencies of signals applied
• Order of Intermod. Product = | C k|
• 3rd order (2F1-F2, 2F2-F1), also 5th and 7th
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9. • Non-ideal wideband linear systems are frequently
treated by expressing the output (Y) of the system as a
power series:
Y a0 a1 X a2 X 2 a 3 X 3 ... a n X n ...
• where X is the total input signal, and the coefficients a
are presumed to be real and independent on X.
• Assume, for simplicity, that input consists of three
elementary signals:
X A(t ) cos( 1 t) B(t ) cos( 2 t ) C(t ) cos( 3 t)
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10. • Some simple calculations will show that the
output of the system Y, in addition to the linearly
transposed input signals, contains the following
spectral components:
• 1st order
Multiplied version of the input signal
a1[ A(t ) cos( 1t ) B(t ) cos( 2 t ) C (t ) cos( 3t )]
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11. • 2nd order
• a) Distorted version of the modulating signals
a2 2 a a
A (t ); 2 B 2 (t ); 2 C 2 (t )
2 2 2
• b) 2nd harmonics
a2 2 a2 2 a2 2
A (t ) cos(2 1t ); B (t ) cos(2 2t ); C (t ) cos(2 3t )
2 2 2
• c) Sum and difference
a 2 A(t ) B(t ) cos( 1 2 )t
a 2 A(t )C (t ) cos( 2 3 )t
a 2 B(t )C (t ) cos( 3 1 )t
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19. Cell clusters
7
3
Various combinations possible
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20. F-D Separation: 3D (Space)
9
4
>8 9 > 27
4
9
1 zone 8 channels
2 zone 27 channels
n zones (n+1)3 channels
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21. Ideal Lattices
• Bound-less, regular, plane lattice
• Each station located at a node
• All nodes occupied (no "holes")
• All stations identical (omnidirectional)
• Uniform propagation (no terrain
obstacles)
• Uniform EM environment
• One set of channels regularly re-used
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22. Equipment deficiency: example
Spectrum “blocked” by typical UHF-TV terrestrial
transmitter due to receiver’s deficiencies (“FCC
20 Taboos”)
18 Area * No. of channels
No of channels denied
16
14
ideal: 1%
12 co-ch: 23%
10 Real other: 77%
8
6
4
2 Ideal
0
0 50 100 150 200 250 300 350
Distance from transmitter, km
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Dixon64
23. Serial-to-Parallel Converter
OFDM
Serial-to-Parallel Converter
F1 Sub-Ch 1 • Orthogonal Frequency Division
Multiplexing (OFDM)
Signal Processing
– The channel is split into a
Demodulation
F2 Sub-Ch 2
number of sub-channels
– Each sub-channel transmits a
part of the original information
– Each sub-channel adjusted to
its environment (S/N)
– Reduces multipath & selective
FN Sub-Ch N fading
– Allows for higher speeds
Digital – Requires smart signal
Modulation processing
– Used in 802.11a(USA),
DTTB(Eu), Hyperplan(Eu),
Delogne P, Bellanger M: The Impact of Signal Processing on an Power Line Coms. standards.
Efficient Use of the Spectrum, Radio Science Bulletin June 1999, 23-28
LeFloch B, Alard M, Berrou C: Coded Orthogonal Frequency Division
Multiplex, Proc of IEEE June 1995, 982-996
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24. TDM
• Time Division Multiplex: A single carrier
frequency channel is shared by a number of
users, one after another. Transmission is
organized in repetitive “time-frames”. Each
frame consists of groups of pulses - time slots.
• Each user is assigned a separate time-slot.
• TDD – Time Division Duplex provides the
forward and reverse links in the same frequency
channel.
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25. TDM TDM Power density
Time-frame
Frequency
Time
Time slot
Example: DECT (Digital enhanced cordless phone) Frame lasts 10 ms, consists of 24 time slots (each 417 s)
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26. SDM
• Space Division Multiple Access controls
the radiated energy for each user in space
using directive antennas
– Sectorized antennas
– Adaptive antennas
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27. CDMA or SS
• Code Division Multiple Access or Spread
Spectrum communication techniques
– FH: frequency hoping (frequency synthesizer controlled by pseudo-random sequence of
numbers)
– DS: direct sequence (pseudo-random sequence of pulses used for spreading)
– TH: time hoping (spreading achieved by randomly spacing transmitted pulses)
– Other techniques
• Hybrid combination of the above techniques (radar and other
applications)
• Random noise as carrier
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28. CDMA - FH SS
Frequency
Power density
CDMA
Bm
Bc
Transmission is organized in time-
Time frequency “slots”. Each link is
Time-frequency slot
assigned a sequence of the slots,
according to a specific code. Used
e.g. in Bluetooth system
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29. DS SS communications
basics
Original
information Spreading
Original signal Spread signal
Propagation effects Transmission Unwanted signals + Noise
Reconstructed
De-spreading information
Spread signal+ Reconstr. signal
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30. SS: basic characteristics
• Signal spread over a wide bandwidth >>
minimum bandwidth necessary to transmit information
• Spreading by means of a code independent of
the data
• Data recovered by de-spreading the signal with
a synchronous replica of the reference code
– TR: transmitted reference (separate data-channel and reference-channel, correlation
detector)
– SR: stored reference (independent generation at T & R pseudo-random identical
waveforms, synchronization by signal received, correlation detector)
– Other (MT: T-signal generated by pulsing a matched filter having long, pseudo-randomly
controlled impulse response. Signal detection at R by identical filter & correlation computation)
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31. DS SS: transmitter
Modulator X Antenna
[A(t), (t)] [g1(t)]
Information
Carrier Modulated signal Spread signal
cos( 0t) S1(t) = A(t) cos( 0t + (t)) g1(t)S1(t)
band Bm Hz band Bc Hz
Bc >> Bm
gi(t): pseudo-random noise (PN) spreading functions that spreads the energy of S1(t) over a bandwidth
considerably wider than that of S1(t): ideally gi(t) gj(t) = 1 if i = j and gi(t) gj(t) = 0 if i j
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32. DS SS-receiver
To demodulator
Correlator
&
antenna X bandpass
filter
Linear
combination g1(t) g1(t)S1(t)
g1(t)S1(t) Spreading g (t) g (t)S (t)
1 2 2
g2(t)S2(t) function ……. S1(t)
……. [g1(t)] g (t) g (t)S (t)
1 n n
gn(t)Sn(t) g1(t) N(t)
N(t) (noise) g1(t) S’(t)
S’(t)
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33. SS-receiver’s Input
W/Hz Unwanted signals
SS s.: g2(t)S2(t); …;
gn(t)Sn(t)
Other s. : S’(t)
Wanted (spread) signal: g1(t)S1(t) Noise: N(t)
Hz
Bc
Signal-to-interference ratio (S/ I)in = S/ [I( )*Bc]
Bc = Input correlator bandwidth
I( ) = Average spectral power density of unwanted signals in Bc
S= Power of the wanted signal
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34. SS-correlator/ filter output
Wanted (correlated) signal: de-spread to its original bandwidth
as g1(t) g1(t)S1(t) = S1(t) with g1(t) g1(t) = 1
Bm Uncorrelated (unwanted) signals
spread & rejected by correlator + noise
g1(t) S’(t); g1(t) N(t); g1(t) gj(t)Sj(t) = 0
as gi(t) gj(t) = 0 for i j
Signal-to-interference ratio
(S/ I)out = S/ [I( )*Bm]
Bc = Input correlator bandwidth
Bm = Output filter bandwidth
I( ) = Average spectral power density of unwanted signals & noise in Bm
Bc S = power of the wanted signal at the correlator output
Spreading = reducing spectral power density
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35. SS Processing Gain =
= [(S/ I)in/ (S/ I)out ] = ~Bc/ Bm
Example: GPS signal
RF bandwidth Bc ~ 2MHz Filter bandwidth Bm ~ 100 Hz
Processing gain ~20’000 (+43 dB)
Input S/N = -20 dB (signal power = 1% of noise power)
Output S/N = +23 dB (signal power = 200 x noise power)
(GPS = Global Positioning System)
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36. SS systems attributes (1)
• Low spectral density of the signal
– LPI: low probability of intercept
– LPPF: low probability of position fix
– LPSE: low probability of signal exploitation
– Privacy
– Covert operations capabilities
– Low interference potential
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37. SS systems attributes (2)
• AJ: anti-jamming/ anti-interference capability
• Security
• Natural cryptographic capabilities
• Multiple-user random access communications
with selective addressing (CDMA)
• High time resolution (~1/B; multi-path suppression)
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38. Summary
• To illustrtae the nature of the multiple access techniques
consider a number of guests at a cocktail party. The aim
is for all the guests to hold an intelligible conversation. In
this case the resource available is the house itself
• FDMA: each guest has a separate room to talk to their
partner
• TDMA: everyone is in a common room and has a limited
time slot to hold the conversation
• FH-CDMA: the guests run from room to room to talk
• DS-CDMA: everyone is in a common room talkim at the
same time, but each pair talks in a different language
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39. Access Control to Radio Resources
• Distributed wireless networks (e.g. packet
radio, ad hoc networks) have no central
control.
• Centralized wireless networks (e.g. WLAN,
Cellular) control the use of radio channel;
various approaches exist
• Slotted systems (e.g. TDMA) require wide
network synchronization for use of discrete
time slots
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41. Packet Radio
• In packet radio access techniques, many
user attempt to access a single channel,
which may led to collisions.
• Protocols aim at limiting collisions
• ALOHA is the oldest, classic protocol,
developed in 1970 in Hawaii as an
extension of TDMA and FDMA
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42. ALOHA
• If 2 or more users transmit at the same time so that receiver
receives more than one packet, the receiver is unable to separate
the packets since they are not orthogonal in time (like in TDMA) or in
frequency (like in FDMA).
• The vulnerable period is the time interval during which the packets
are susceptible to collisions with transmissions from other users
Transmitter 1 Packet B Packet C
Transmitter 2 Packet A
t1 T1+2
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43. • In pure ALOHA, the vulnerable period is 2 packet durations. A user
transmits whenever it has a packet to deliver. If no acknowledgment
(ACK) is received, the user waits a random time and retransmit the
packet. The throughput is T = Re-2R, R being the normalized channel
traffic in Erlangs (Tmax = 0.184 at R = 0.5)
• In slotted ALOHA, time is divided into equal time slots of length
greater than the packet duration. The users have synchronized
clocks and transmit messages only at the beginning of a new time
slot. This prevent partial collisions where one packet collides with a
portion of another. The vulnerable period is only one packet
duration. The throughput is T = Re-R (Tmax = 0.368 at R = 1)
• ALOHA protocols do not listen to the channel before transmission,
and do not exploit information about the other users.
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44. • Carrier Sense Multiple Access (CSMA)
protocols base on monitoring the channel.
If the channel is idle (no carrier is
detected), then the user is allowed to
transmit. Important are detection delay
and propagation delay.
• Reservation protocols – certain packet
slots are assigned with priority
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45. References
• Coreira LM, “Wireless Flexible
Personalized Communications”, J Wiley
• Dunlop J, Smith DG, “Telecommunication
Engineering”, Chapman & Hall
• Reed JH, “Software Radio”, Prentice Hall
• Taub H, Shilling DL, “Principles of
Communication Systems”, McGraw Hill
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